Field of the Invention
The present invention relates to an apparatus for detecting a variation abnormality in an air-fuel ratio between cylinders in an internal combustion engine having a plurality of cylinders.
Description of the Related Art
Generally, in an internal combustion engine provided with an exhaust gas purification system that uses a catalyst, it is necessary to control a mixture ratio between air and fuel, that is, an air-fuel ratio, of an air-fuel mixture to be burned in the internal combustion engine in order to purify harmful components in exhaust gas using the catalyst with high efficiency. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and feedback control is performed such that the air-fuel ratio detected by the air-fuel ratio sensor follows a predetermined target air-fuel ratio.
On the other hand, in an internal combustion engine having a plurality of cylinders, that is, a so-called multi-cylinder internal combustion engine, air-fuel ratio control is usually performed over all the cylinders by using the same control amount, and thus an actual air-fuel ratio may vary among the cylinders even when the air-fuel ratio control is performed. In this case, a small variation can be accommodated by air-fuel ratio feedback control, and harmful components in the exhaust gas can be purified by the catalyst. Therefore, such a small variation does not influence exhaust emissions and does not matter.
However, if a fuel injection system of one or some of the cylinders or a valve system of an intake valve goes out of order, for example, the air-fuel ratio varies greatly among the cylinders. This causes a problem of deterioration of exhaust emissions. It is desirable to detect such a large air-fuel ratio variation that deteriorates exhaust emissions as an abnormality.
For example, conventionally there is a suggestion of detecting occurrence of a variation abnormality in an air-fuel ratio between cylinders, that is, an air-fuel ratio imbalance, by comparing a detected air-fuel ratio change rate based on the output of an air-fuel ratio sensor provided in the exhaust passage with a determination threshold (see, for example, International Publication No. WO2011/070688).